Many bacteria establish pathogenic or symbiotic interactions with higher organisms. We study the nutritionally mutualistic interaction or Rhizobium meliloti with a host legume, alfalfa. This host-microbe association proceeds by a ordered series of steps, which are unique and are highly specific to the interaction of particular species. The early steps of the interaction are controlled by a series of bacterial nodulation (nod) genes, whose expression is induced by a small flavonoid compound, luteolin, produced by the host. In addition, induction of nod genes depends on a protein, NodD. In R. meliloti, three nodD genes encode regulatory proteins with distinguishable sizes and properties. The roles in induction of the flavone (luteolin) inducer, the regulatory upstream DNA sequences, the NodD protein, and RNA polymerase will be studied. Luteolin will be labelled and its receptor protein identified. NodD and NodD3 will be purified and characterized to determine the basis for NodD mediating flavone induction of nod genes, while NodD3 causes constitutive nod gene expression. RNA polymerase will be purified, and specialized sigma factors will be isolated and characterized for effects on nod gene expression. Deletion analysis will be used to indicate the roles of sequences upstream of inducible nod genes. The existence of regulatory genes besides nodD will be tested genetically, and the products of such genes will be characterized. The specific hypothesis that NodD binds flavone, and activates transcription through direct interaction with DNA, will be analyzed in detail.